A biosensor (or biological sensor) is an analytical device including a biological element and a transducer that converts a biological response into an electrical signal. Certain biosensors involve a selective biochemical reaction between a specific binding material (e.g., an antibody, a receptor, a ligand, etc.) and a target species (e.g., molecule, protein, DNA, virus, bacteria, etc.), and the product of this highly specific reaction is converted into a measurable quantity by a transducer. Other sensors may utilize a non-specific binding material capable of binding multiple types or classes of molecules or other moieties that may be present in a sample, such as may be useful in chemical sensing applications. The term “functionalization material” may be used herein to generally relate to both specific and non-specific binding materials. Transduction methods used with biosensors may be based on various principles, such as electrochemical, optical, electrical, acoustic, and so on. Among these, acoustic transduction offers a number of potential advantages, such as being real time, label-free, and low cost, as well as exhibiting high sensitivity.
An acoustic wave device employs an acoustic wave that propagates through or on the surface of a functionalization (e.g., specific binding) material, whereby any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Presence of functionalization material on or over an active region of an acoustic wave device permits an analyte to be bound to the functionalization material, thereby altering the mass being vibrated by the acoustic wave and altering the wave propagation characteristics (e.g., velocity, thereby altering resonance frequency). Changes in velocity can be monitored by measuring the frequency, amplitude-magnitude, or phase characteristics of the acoustic wave device, and can be correlated to a physical quantity being measured.
In the case of a piezoelectric crystal resonator, an acoustic wave may embody either a bulk acoustic wave (BAW) propagating through the interior of a piezoelectric material, or a surface acoustic wave (SAW) propagating on the surface of the piezoelectric material. SAW devices involve transduction of acoustic waves (commonly including two-dimensional Rayleigh waves) utilizing interdigital transducers along the surface of a piezoelectric material, with the waves being confined to a penetration depth of about one wavelength. Typically, BAW devices are fabricated by micro-electro-mechanical system (MEMS) fabrication techniques owing to the need to provide microscale features suitable for facilitating high frequency operation. BAW devices typically involve transduction of an acoustic wave using electrodes arranged on opposing top and bottom surfaces of a piezoelectric material. In a BAW device, three wave modes can propagate, namely: one longitudinal mode (embodying longitudinal waves, also called compressional/extensional waves), and two shear modes (embodying shear waves, also called transverse waves), with longitudinal and shear modes respectively identifying vibrations where particle motion is parallel to or perpendicular to the direction of wave propagation. The longitudinal mode is characterized by compression and elongation in the direction of the propagation, whereas the shear modes consist of motion perpendicular to the direction of propagation with no local change of volume. Longitudinal and shear modes propagate at different velocities. In practice, these modes are not necessarily pure modes as the particle vibration, or polarization, is neither purely parallel nor purely perpendicular to the propagation direction. The propagation characteristics of the respective modes depend on the material properties and propagation direction respective to the crystal axis orientations. The ability to create shear displacements is beneficial for operation of acoustic wave devices with fluids (e.g., liquids) because shear waves do not impart significant energy into fluids.
Certain piezoelectric thin films are capable of exciting both longitudinal and shear mode resonance, such as hexagonal crystal structure piezoelectric materials including (but not limited to) aluminum nitride (AlN) and zinc oxide (ZnO). To excite a wave including a shear mode using a piezoelectric material arranged between electrodes, a polarization axis in a piezoelectric thin film must generally be non-perpendicular to (e.g., tilted relative to) the film plane. In biological sensing applications involving liquid media, the shear component of the resonator is used. In such applications, piezoelectric material may be grown with a c-axis orientation distribution that is non-perpendicular relative to a face of an underlying substrate to enable a BAW resonator structure to exhibit a dominant shear response upon application of an alternating current signal across electrodes thereof.
Fabricating a BAW resonator device may involve depositing an acoustic reflector over a substrate, followed by deposition of a bottom side electrode, followed by growth (e.g. via sputtering or other appropriate methods) of a piezoelectric material, followed by deposition of a top side electrode. Growth of the piezoelectric material could be by chemical vapor deposition (CVD), reactive RF magnetron sputtering (e.g., of Al ions in a nitrogen gas environment), etc. These techniques are capable of forming layers that are uniformly thick (e.g., piezoelectric material via sputtering), although some layers may have portions of differing heights depending on the topography of an underlying material deposition surface. For example, a bottom side electrode may not cover an entirety of the underlying acoustic reflector, such that a material deposition surface including the foregoing layers over a substrate may include bottom side electrode material that is slightly raised with respect to a top surface of the acoustic reflector. Upon application of a uniformly thick piezoelectric material over the material deposition surface, portions of the piezoelectric material positioned over the bottom side electrode will be raised relative to other portions of the piezoelectric material that are not overlying the bottom side electrode.
Modes of vibration in a solidly mounted resonator (SMR) type BAW devices are determined based on an assumption that the piezoelectric material is an infinite plate defined by dimensions of the electrodes arranged over and under the piezoelectric material forming an active region. Outside the active region, the BAW resonator device is mechanically clamped (e.g., mechanically restrained from freely moving) in the lateral direction due to presence of piezoelectric material bordering a periphery of the active region. For a BAW resonator device vibrating with mixed longitudinal and shear modes, this mechanical clamping has the potential to degrade a desired shear mode response in a plane of the piezoelectric material. In particular, such mechanical clamping tends to damp shear mode vibrations (e.g., shear mode response, shear displacement, etc.) of the active region, thereby limiting detection sensitivity and performance of the BAW resonator device.
Accordingly, there is a need for improved acoustic wave devices capable of enhanced shear mode vibrations, such as for biosensing or biochemical sensing applications, that overcome limitations associated with conventional devices.